Probeheads of the kind mentioned above are generally known, e.g. to carry out measurements on anisotropic samples by means of magnetic resonance. Units for that purpose may, for example, be found in the textbook by Hausser, K. H. et al. "NMR fur Mediziner und Biologen, Springer Verlag 1989".
In the brochure "NMR Microscopy", published by applicant in 1991, a measuring method is described denoted by the acronym STRAFI (Stray Field Imaging). This measuring method is used to record two and three dimensional image representations of solid state samples. Since it is known that solid state samples have very short spin-spin relaxation times, and therefore large line widths, the field gradients, which can be applied with conventional nuclear resonance devices, are not sufficient, to achieve a sufficiently good slice selection.
The above mentioned STRAFI-method makes use of the fact that magnetic coils as used in nuclear resonance technique, exhibit a stray field at their edge which is strongly inhomogeneous, e.g. it shows a very strong field decrease and can therefore be used as a gradient field with strong field gradient. If a solid state sample is brought into this stray field at the edge of the magnet coil, this sample is located in a sufficiently pronounced gradient field whose field gradient is so large that the required slice selection is achieved with the available line width.
Since the stray field is a quantity predetermined by the apparatus, it is, however, impossible to switch the gradient as with the usual measuring gradients in nuclear resonance devices. Therefore, with the STRAFI method one is forced to move the solid state sample in the stray field in a predetermined manner, in order to facilitate the desired two or three dimensional image representation.
In the above mentioned brochure an experiment is described where solid state samples were brought into the stray field of a superconducting magnetic coil, whose field gradients at the location of the sample were in the order of 40 to 80 T/m, with a rated field of the magnetic coil between 4.7 and 9.4 T.
In order to achieve the desired two and three dimensional image representations of the sample, the sample was moved by computer controlled step motors on the one hand in axial direction and on the other hand turned about one or two axes perpendicular to the magnet axis, this, too, by means of step motors. Further details of the apparatus used cannot be taken from the above mentioned brochure.
If in such experiments a sample shall be turned about one or even about two axes, this imposes special requirements on the design of the transmission/detection unit used, in view of the extremely confined spatial circumstances (to reach a high filling factor).
If, for example, the transmission/detection unit is designed as a Helmholtz radio frequency coil system, consisting of two partial coils comprising a central gap through which the sample may be manipulated, this has the consequence of an insufficient homogeneity of the radio frequency field. Moreover, in this case, because of the relatively high electric field strengths, the tuning and coupling conditions change, in particular if the sample is turned by tilting same in the gap plane, since a Helmholtz configuration does not act in a strictly rotation symmetrical way because of the necessary connection of the two partial coils. As a consequence of such a configuration, the measuring result is distorted by systematic influences, if the sample is turned.
Therefore, it is the object of the invention to improve a probehead of the above mentioned kind in that the sample turning is possible in a constructionally simple way wherein the homogeneity of the radio frequency field and/or the tuning or coupling conditions, respectively, should not suffer by turning of the sample.
This object is on the one hand achieved according to the invention in that the transmission/detection unit is a split ring resonator, which is, preferably together with the sample, turnable about its resonator axis and which can inductively be coupled to via coupling means, wherein the coupling means is in a fixed spatial position.
The object is further achieved according to the invention in that the transmission/detection unit is a split ring resonator which is located inside the probehead in a fixed spatial position, which split ring resonator exhibits a circumferential narrow slit in its central plane.
In this way the object of the invention is completely achieved.
Surprisingly the homogeneity of a split ring resonator has turned out to be exceptionally good, in particular with the measuring frequencies around 200 MHz, which presently are usually used for imaging methods on solid state samples. A further advantage of such a resonator with low L/C-ratio is that only comparatively low electric field strengths are present with the consequence that heating of the sample and detuning of the resonance circuit by the sample are kept at a negligible low level. Further it turned out that the radio frequency field of a split ring resonator remains widely unchanged if the split ring resonator is coupled via coupling means coaxial to the resonator axis and if the split ring resonator is turned about the resonator axis relative to the coupling means.
In contrast to the possibilities of a Helmholtz configuration described further above, where the partial coils as well as the coupling means are arranged in a fixed spatial position and unchanged with respect to each other, the invention uses a trick where the coupling means stays in a fixed spatial position inside the probehead and the split ring resonator turns together with the sample relative to the coupling means.
A split ring resonator has the further advantage that a sample located inside the split ring can easily be manipulated since the sample can be turned about a further axis perpendicular to the resonator axis without a considerable distortion of the field distribution in the split ring. At this point it should again be mentioned that the turning of the sample about the first axis is already effected by the turning of the split ring itself, so that during this first turning no relative movement of sample and transmission/detection unit occurs. Only during the turning about a second axis perpendicular to the resonator axis, a relative movement of sample and split ring is necessary, this turning may, however, as mentioned above, be performed with only a very minor influence onto the field distribution inside the split ring.
The feature to use a split ring resonator which is spatially fixed inside the probehead as a transmission/detection unit, which split ring resonator has in its central plane a circumferential narrow slit, has the advantage that coupling may also be effected capacitively, or that in the case of inductive coupling the coupling components do not move. This may in certain cases counterbalance the disadvantage that in this case the sample cannot be turned about the resonator axis by full 360.degree..
In a preferred embodiment of the invention the coupling means are in a fixed spatial position with respect to the split ring resonator and coaxial to the resonator axis.
This feature has the advantage that during the use of the apparatus according to the invention, the coupling and tuning conditions change as little as possible.
In a further preferred embodiment of the invention the coupling means are an essentially circular coupling loop.
This feature has the advantage that the rotational symmetry of the configuration leads on the whole to an extremely low influence onto the field distribution as well as the coupling conditions if the split ring is turned relative to the coupling means.
In a further preferred embodiment of the invention the sample inside the split ring resonator can additionally be turned about a second axis which is tilted with respect to the resonator axis and which is preferably perpendicular to this axis.
This feature is particularly advantageous if three dimensional image representations shall be produced and it has already been mentioned that this additional turning of the sample about a second axis is possible without problems for a split ring resonator.
In a practical embodiment of the apparatus according to the invention, the split ring resonator comprises a split ring being arranged in, preferably supported by, a support ring with a larger diameter, wherein the support ring is mounted in guiding roller bearings and connected to drive means for the turning.
This feature has the advantage that the electrical function of the split ring is separated from the mechanical function of turnability, so that in order to turn the split ring, drive means may be applied with no consideration for the special design of the split ring and that the split ring may easily be exchanged if necessary, e.g. for adaption RF-wise to a new measuring frequency or geometrically to a new sample.
In a preferred improvement of the embodiment the support ring comprises a sample holder pointing radially inward and being turnable about the second axis and which can be operated from the outer circumference of the support ring.
This measure has the advantage that the turning of the sample about the second axis, perpendicular to the resonator axis, can be adjusted in a simple way from the outside of the support ring.
This may be achieved particularly well in an improvement of this embodiment where the support ring comprises a reference mark in such a way that the support ring may be locked in a well defined turning position about the resonator axis, whereby in this turning position operating means mounted inside the probehead can be connected to the turnable sample holder.
This feature has the advantage that in a first step by a full turning of the support ring about 360.degree. a first group of measuring values can be recorded wherein the sample is in a particular turning position about the second rotation axis. When this defined turning position (zero position) is reached again, the sample can be turned about the second turning axis in a defined way by an angular increment by means of the operation means mounted in the probehead. Subsequently, the support ring is again turned about 360.degree. in order to record a second group of measuring values for defined angular positions of the support ring, etc.
In a further group of embodiments the split ring comprises one or more capacitors in its gap, wherein the capacitor is preferably connected to the support ring by an insulating support being radially arranged inside the support ring.
This feature has the advantage that on the one hand the split ring can be optimized with respect to its electrical properties (reduction of losses in the electric field) by providing the capacitor, on the other hand the thus required capacitor is also used mechanically to provide for necessary support of the split ring in the support ring.
Further advantages of the invention will appear from the specification and the attached drawing.
It is understood that the features that have been mentioned before and that will be described hereafter may be used not only in the stated combinations, but also in any other combination or each alone, without departing from the scope of the present invention.
An embodiment of the invention is represented in the drawing and will be described in more detail in the following description.